1. Field of the Invention
This invention relates to electronic fuel control systems for gas turbine aeroengines, and in particular to enhancement of such systems for the purpose of improving the reliability of relighting of the engines.
To relight a turbofan or similar engine, the forward speed of the aircraft is used to induce "windmilling" of the engines' bladed rotors due to impingement of the air stream on them. This provides mechanical power for engine-driven accessories such as fuel pumps and enables a relight to be attempted using continuously operated electrical igniters in the combustor.
2. Description of the Related Art
Prior to the present invention, relighting gas turbine aeroengines in flight was tackled by simply causing the fuel system to provide a nominally constant low fuel flow to the combustion chambers while the speed of the engine's high pressure spool increased. Upon achievement of a satisfactory light-up, the system switched to a normal acceleration schedule of increasing fuel flow to enable engine power to be built up to a satisfactory level. In order to cope with the effect of low air temperatures, a switch was provided to enable a higher constant fuel flow for relight at air temperatures below a certain predetermined value.
Previously, such a relight fuel flow schedule was implemented in a hydromechanical fuel system, but with the advent of full authority digital electronic systems, the same type of relight schedule was merely transferred to them and implemented in software rather than hardware.
In the event of one or more of a civil aircraft's engines being extinguished, the ability of the engines to relight quickly and reliably is an important safety consideration, particularly so in the case of extended range operations (EROPS). Unfortunately, relightability of gas turbine aeroengines decreases with altitude. In general the fuel flow needed for ignition gets progressively higher with altitude, due to low air pressure at the entrance to the combustion chamber, but the precise fuel flow required for a satisfactory light-up becomes less predictable as altitude increases. Consequently, in the unlikely event of an engine being extinguished at high altitude, existing fuel systems have made it necessary for the aircraft to descend to an altitude more favourable for relighting. This manoeuvre is undesirable from a safety point of view, and the need for the aircraft to climb back up to the desired operating altitude imposes a fuel consumption penalty.